1. Field of the Invention
The present invention relates to a transmission control system responsive to a shift control signal for effecting a gear change or shift by disengaging a synchronizing clutch for a present gear position, achieving a neutral gear position, and then engaging a synchronizing clutch for a next gear position.
2. Description of the Prior Art
Synchronizing clutches are clutches engageable by meshing mechanical components. They include a clutch with a synchromesh mechanism, a clutch with a roller synchronizing mechanism, and a clutch with a dog tooth mechanism.
There are known automobile transmissions which control gear changes or shifts with controlled engagement and disengagement of synchronizing clutches. One transmission which comprises synchronizing clutches with roller synchronizing mechanisms is disclosed in U.S. Pat. No. 4,817,451, for example.
Japanese laid-open patent publication No. 61-94830 discloses an automatic transmission for automatically carrying out gear changes with synchronizing clutches. When input and output members of a synchronizing clutch rotate out of synchronism with each other, an output torque transmitted through the synchronizing clutch acts to prevent the clutch from being engaged or disengaged, resulting in an increased force required to bring the gears out of mesh with each other. It is therefore necessary to disengage or engage the synchronizing clutch when the torque transmitted therethrough is eliminated and hence the force required to bring the gears out of mesh with each other is eliminated.
When the automatic transmission disclosed in Japanese laid-open patent publication No. 61-94830 is to make a gear change, a transmission actuator applies a force to a synchronizing clutch for a present gear position, which force is of a minimum level required to bring this synchronizing clutch into a neutral gear position. At the same time, the throttle valve of an engine coupled to the automatic transmission is gradually closed to lower the output power of the engine. When the output torque transmitted from the engine through the synchronizing clutch for a present gear position is substantially eliminated and hence the force required to disengage this synchronizing clutch, i.e., when drive and driven members of the synchronizing clutch are free of loads, the synchronizing clutch is brought into the neutral gear position of their own accord under the force imposed by the transmission actuator.
If the output torque of the engine which is transmitted through the synchronizing clutch for a present gear position abruptly dropped to a zero or non-load condition and then to a negative level, then since the output torque of the engine would be eliminated for a very short period of time, the gears could not be brought out of mesh with each other, and hence the neutral gear position could not be achieved. To avoid this drawback, the throttle valve of the engine is gradually closed to lower the output torque of the engine gradually for allowing the gears to be brought reliably out of mesh with each other.
According to such a control process, however, because the throttle valve of the engine starts being gradually closed after a shift command has been issued, it takes a certain period of time until the synchronizing clutch for a present gear position is disengaged to reach the neutral gear position after the shift command has been issued. Therefore, the operation of the transmission to accomplish a gear change is relatively slow.
Furthermore, the above control is effective only when the output torque of the engine is of a positive value, i.e., the engine is being accelerated, at the time the transmission starts to effect a gear change. That is, only when the output torque of the engine is of a positive value under an accelerating condition, the throttle valve is gradually closed to lower the output torque, and the neutral gear position is reached when the output torque substantially drops to a zero level, i.e., the transmission is subject to no load. If the output torque were of a negative value, i.e., the engine were being decelerated, or the output torque were zero, i.e., the transmission were under no load, at the time the transmission starts to effect a gear change, then closing the throttle valve would additionally lower the output torque of the engine. The output torque would thus not drop to zero, failing to disengage the synchronizing clutch for a present gear position.
In view of the above shortcomings of the conventional transmission control system, it has been proposed, as disclosed in Japanese patent application No. 2-401192, to employ a map of throttle valve openings and engine rotational speeds, the map including an accelerating region where the engine output torque is positive and a decelerating region where the engine output torque is negative, and to determine whether the engine is being accelerated or decelerated using the map at the time the transmission starts to make a gear change. If the engine is determined as being accelerated, then the engine output power is lowered until a non-load condition is reached, i.e., until the engine output torque transmitted through the synchronizing clutch for a present gear position drops to zero, when the synchronizing clutch is disengaged. Conversely, if the engine is determined as being decelerated, then the engine output power is increased until a non-load condition is reached, when the synchronizing clutch for a present gear position is disengaged.
However, if the engine condition is in the boundary between the accelerated and decelerated conditions at the time the transmission starts to make a gear change, i.e., if the engine output torque transmitted through the synchronizing clutch for a present gear position is in the vicinity of the zero level, then it is difficult to determine whether the engine is being accelerated or decelerated. When this happens, controlling the transmission for a gear change may become unstable.
The unstable transmission control is problematic in that since the transmission may not effect a downshift, for example, when a downshift command is issued, the automobile may not be decelerated even though the driver wishes to decelerate the automobile through a downshift. As a result, the automobile tends to lose stability while it is running.